High-power visible light sources having high monochromaticity have been demanded as light sources for an image display device which displays image on a screen. When laser light sources of three primary colors, i.e., red, green, and blue are used as these light sources, the reproducible color range can be largely broadened because of their strong monochromaticity. As for the red or blue light source, a high-power laser light source can be configured using a small-size and high-efficiency semiconductor laser. Further, as for the green light source, a small-size, high-efficiency, and high-power laser source can be realized by using a green laser due to a second-order harmonic generation which uses a semiconductor laser as an excitation light source (hereinafter referred to as a SHG green laser).
By the way, in order to use a semiconductor laser as a laser emission device which is a light source for an image display device, a W-class light output power is required for each of red, green, and blue. Particularly for red and blue, it is necessary to configure the laser emission device using at least five to ten or more semiconductor laser elements, and it is demanded that the laser emission device as a whole should have a high efficiency, a low power consumption, and a long lifetime by devising the way of using these plural laser elements.
As an example of such laser emission device, there has been disclosed a technique of extending the lifetime of a semiconductor laser array which outputs plural beams, not in a display application for which high luminance is required but in a beam scanning device for which high resolution is required (for example, refer to Patent Document 1). FIG. 16 is a block diagram illustrating a functional configuration of such beam scanning device. To be specific, in FIG. 16, a usable LD (laser) selection means 1 judges the emission efficiencies of the respective laser diodes or whether the laser diodes are used or not, according to the contents such as control signals for drive-controlling the laser diodes or data as to whether the laser diodes have been used or not for the previous job or printing of the previous page, which contents are stored in a memory 3, and selects laser diodes to be used next from among the laser diodes in the semiconductor laser array 5. An LD control means 2 drive-controls the laser diodes which are selected by the usable LD selection means 1 and a printing mode input means 4, and controls the light amount of the semiconductor laser array 5 at start-up. In this way, the laser diodes having less deteriorations are preferentially selected according to such as the levels of their emission efficiencies, and thereby the respective laser diodes are equally used. As the result, the lifetime of the semiconductor laser array 5 is extended.
On the other hand, Patent Document 2 discloses a laser light source which measures the temporal deterioration rates of the output efficiencies of the respective semiconductor lasers, and performs control so as to make the light output powers of the respective semiconductor lasers different from each other according to the measured values, thereby to reduce the light output power of the semiconductor laser having relatively large temporal deterioration rate to extend the live thereof, thereby extending the cycle of replacement or maintenance for the laser light source.
Further, Patent Document 3 discloses, as a configuration which hardly deteriorates the lifetime, a configuration which multiplexes the light beams emitted from plural semiconductor laser elements to obtain a single laser output, and controls the laser outputs from the plural semiconductor elements to a rated value or below.
Furthermore, Patent Document 4 proposes a method of halting current injection into a faulty laser light source, and balancing the light output power using other laser light sources. Patent Documents 5, 6, and 7 propose a method of preparing a spare element for a faulty or abnormal laser light source, and compensating the output power by using the spare element instead of the faulty or abnormal laser light source, thereby to stabilize the power of the laser light source.
Patent Document 1: Japanese Published Patent Application No. Hei.8-171061
Patent Document 2: Japanese Published Patent Application No. 2003-338660
Patent Document 3: Japanese Published Patent Application No. 2004-214225
Patent Document 4: Japanese Published Patent Application No. 2004-279943
Patent Document 5: Japanese Published Patent Application No. 2002-120395
Patent Document 6: Japanese Published Patent Application No. 2001-267681
Patent Document 7: Japanese Published Patent Application No. Hei.5-343809